High-impact polystyrene is typically a two-phase polymer consisting of rubber-like particles embedded in a polystyrene matrix. Typically, polystyrene is trapped (occluded) in the rubber-like polymer particles.
When the amount of rubber-like polymer employed is held constant, and the amount of the polystyrene occluded (trapped) within the rubber-like polymer particles is increased, the size of the rubber-like polymer particles increases providing a high-impact polystyrene having improved properties. When the amount of rubber-like polymer employed is decreased and the amount of polystyrene occluded within the rubber-like polymer particles is increased, the properties of the high-impact polystyrene remain constant. It is desirable to provide a high-impact polystyrene that has improved properties without increasing the amount of rubber-like polymer needed or a high-impact polystyrene that has equivalent properties using less rubber-like polymer.
It is important to have crazes, initiated by rubber particles, to obtain efficient use of rubber in the toughening of polystyrene. Crazes are microcracks which are formed to relieve stress in a deformed part. When a low number of crazes are initiated it is more likely that one of the crazes will become a large crack and lead to early failure of the deformed part. Rubber phase characteristics (particle size, size distribution, volume, polystyrene occlusions, etc) can affect craze formation and are important in providing reinforcement of the polystyrene matrix in high-impact polystyrene. The volume of the polystyrene occlusions is also important to high-impact polystyrene craze initiation. Crazing is initiated when rubber particles are near the surface of the part under stress. Large polystyrene occlusions are desirable for efficient craze initiation.
In addition, the rubber-like polymer particles must be anchored in the polystyrene matrix. This is accomplished by grafting (anchoring) the polystyrene onto the rubber-like polymer. The resulting graft copolymer resides at the interface between the rubber-like polymer and the polystyrene.
Although some grafting of the rubber-like polymer and polystyrene may occur without the use of a grafting catalyst or initiator, a grafting catalyst or initiator increases the formation of a graft. However, the presence of an initiator can have an adverse effect on the polymerization of the styrene monomer. For example, when a grafting catalyst or initiator, such as peroxide, is used to control grafting the peroxide can react with mercaptans that are typically used in the polymerization process to control the molecular weight of the polystyrene thereby resulting in polymers of undesirable molecular weight. Therefore, it is desirable to provide a process for making high-impact polystyrene where the grafting catalyst does not destroy the molecular weight modifier.
Accordingly, it is important to achieve a balance between the amount of rubber-like polymer, the size and polystyrene content of the occlusions in the rubber-like polymer particles, and the strength of the graft between the dispersed and continuous phase in order to improve the morphology of the high-impact polystyrene.
Japanese Kokai Patent SHO 63-113009 (1988) discloses a process sequence wherein a blending type reactor is used to prepolymerize styrene monomer and a rubber-like polymer forming a prepolymerized solution. The prepolymerized solution is fed into a mixing apparatus containing an agitation device, such as a propeller, where the prepolymerized solution is mixed with a circulating polymer solution. Phase inversion occurs in the mixing device.
In the discussion in Japanese Kokai Patent SHO 63-113009 (1988), Japanese Kokai Patent SHO 59-10513 (1984) is described as disclosing a process for making an impact-resistant polystyrene in which "the first flow from pre-graft treatment is blended with the second flow made up of polymerized styrene solution to granulize the rubber-like substance."
Japanese Kokai Patent SHO 63-118315 (1988) discloses a process wherein a first and second complete mixing-type reaction container are connected in series with a plug flow-type reaction container. Phase inversion occurs in the complete mixing-type reaction containers.
A German reference, DE No. 4,017,281 discloses a process wherein polybutadiene rubber-like polymer and polymerized polystyrene are prepared in completely mix-type reactors and are then fed into a mixer such as an in line-mixer equipped with a stirrer or a reactor of the completely-mixed type.
The present invention differs from the above references in a critical aspect. In the present invention, phase inversion occurs in a quiescent reaction zone. In the above references, phase inversion occurs in a mixing apparatus or a complete mixing-type reaction container. When mixing or stirring is used during phase inversion, the mixing will break or disrupt the rubber-like polymer particles resulting in a particle size distribution that is broad, which is undesirable.
It is a general object of the present invention to provide an improved process for making an improved high-impact polystyrene.
It is an object of the present invention to provide an improved process for making a high-impact polystyrene in which the size of the rubber-like polymer particles is relatively large and relatively evenly distributed, the content of polystyrene occlusions is relatively large, and the rubber-like polymer particles are relatively strongly grafted to the polystyrene.
It is a further object of the present invention to maximize the separation of the process of grafting the rubber-like polymer and the polystyrene from the process of polymerizing the styrene monomer, to thereby separate incompatible feed components and minimize undesirable side reactions.
It is a still further object of the present invention to maximize the use of the rubber-like polymer by reducing the amount of rubber-like polymer used in the particles while, at the same time, not decreasing the morphology of the high-impact polystyrene.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims.